There is a need for the production of optical record carriers of high capacity. Therefore, optical scanning devices using a relatively short wavelength radiation beam, for example a radiation beam of 400 nm, a high numerical aperture (NA) objective lens system, at least 0.7 and for example NA=0.85, and a thin protective cover layer, for example 80 μm thickness, are desirable. Furthermore, the capacity can be increased by providing a dual layer disk. At the wavelength and NA mentioned, a layer separation of at least 20-30 μm is desirable in order to reduce the coherent cross talk to an acceptable level. Without compensating measures, refocusing from one layer to the other results in spherical aberration, generating a wavefront error of 200-300 mλ (rms), which deteriorates the resolution of the optical spot formed.
It is known to mechanically adjust the spacing of the two, or more, lens elements of a compound objective lens, in order to provide spherical aberration compensation. Another method of compensation is by mechanically adjusting the position of a collimator lens with respect to the radiation source, so that the radiation beam impinges on the objective lens as a convergent, or divergent, instead of collimated, beam. Each of these methods compensates spherical aberration generated in the optical system of the scanning device, to at least approximately cancel out that generated in the optical disk being scanned.
However, using mechanical actuators to provide spherical aberration compensation, particularly when a separate mechanical actuator is used to provide focus control, is relatively complex and therefore increases the cost of manufacture of the scanning device.
A further known optical scanning device is described in WO-A-124174, in which a radiation beam is passed through a twisted nematic (TN) liquid crystal cell which selectively rotates the polarization of incident light by 90°. The beam is then passed, when in a convergent state, through a birefringent plate to produce spherical aberration therein. The birefringent plate produces different amounts of spherical aberration depending on the state of the TN cell, to compensate for the different information layer thicknesses.
EP-A-08605037 A1, and an article Applied Optics volume 38 (1999) pp 3778-3786 by R. Katayama, describes a phase structure which is used to make an objective lens designed for scanning DVD record carriers also suitable for scanning CD record carriers. In general, DVD record carriers are designed for being scanned with a radiation beam of a wavelength and numerical aperture different from that used for scanning a previous generation of record carrier such as CD. The phase structure consists of stepped non-periodic annular zones, such that each zone gives rise to a phase step which is equal to a multiple of 2π for the DVD wavelength (660 nm), so that the phase structure has no effect at this wavelength. For CD read out, however a different wavelength is used (785 nm). Consequently, the stepped phase profile results now in phase steps which no longer are equal to a multiple of 2π. By proper design of the steps heights and zone width, the phase introduced by the phase structure in the CD case reduces the wavefront aberration caused by the disk thickness difference to below the diffraction limit. The structure is capable of reducing the wavefront aberration for two discrete wavelengths.
JP-A-2001-174614 describes a diffractive device for an optical head for an optical scanning device capable of operating at two different wavelengths. The diffractive device includes a grating formed of a birefringent material embedded in a material having a uniform refractive index. The birefringent material is arranged in a periodic structure, i.e. one which regularly repeats across the element. For one wavelength, the device transmits light at one polarization without diffraction and diffracts light at the orthogonal polarization. For the second wavelength, light is transmitted without diffraction at both polarizations. One drawback is that the production of the periodic phase structure, i.e. the grating, is relatively complex due to the large number of elements within the structure. Furthermore, due to the diffraction, a certain amount of the input light is wasted, which is undesirable.